As electronic equipment becomes smaller and lighter, it is necessary to reduce the width of wiring lines and the diameter of the via holes which connect layers in multi-layer printed wiring boards. It is very difficult to use mechanical drilling to form holes below about 200 .mu.m diameter in an industrial scale, and lasers have been used to make such small holes.
A carbon dioxide laser can form holes at high speed in organic substances, such as epoxy resin and polyimide resin. Such lasers have been widely used in making printed wiring boards. Forming holes in copper foil is difficult, however, because the copper foil reflects the laser beam. To solve this problem, as disclosed in Japanese Patent Publication No. 4-3676, a hole is etched through the copper foil having the same diameter as a via hole. Then, the laser beam is used to form the hole through the organic substrate, with the beam diameter being larger than that of the via hole. When such a process is used, additional plating must be applied to the copper foil as well as in the via holes. Consequently, the thickness of the outer layer of copper layer is the sum of the thickness of the copper foil itself and the thickness of the plated copper, and it is not easy to form fine pitch wiring lines. Furthermore, it is not easy to etch a hole in an outer wiring with the hole aligned with an inner pad, because highly accurate alignment is required.
In another process, the faces of an inner layer board which have wiring patterns are coated with an insulating resin, holes are formed in the resin by the laser beam, and then the resin surfaces are directly plated with copper to form an outer copper wiring. Only a single layer of copper is deposited. In this process, however, it is necessary to roughen the insulating resin surfaces to obtain acceptable adhesion strength between the plated copper and the insulating resin. Roughening of the insulating resin surfaces often cannot provide sufficient adhesion strength between the copper layer and the insulating resin.